We now consider directed graphs (a.k.a. digraphs) and trails
therein. It is important to note that given two vertices \(u\)  and \(v\)  in a
digraph, if the arcs \((u,v)\)  and \((v,u)\)  both exist, they are considered to be
different arcs, and therefore they can both appear in the same trail.
(This follows the definitions used in [1]}.) In
particular, this means that a directed trail in a symmetric digraph – symmetry
means that the arc \((u,v)\)  is present if and only if \((v,u)\)  is – does not
necessarily induce a trail in the corresponding undirected graph.
